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Original question:

Float casting in IRQ (RM46)

RM46L430: Using floating point registers in IRQ and FIQ (RM46)

Falk Sobe
Intellectual 335 points
Part Number: RM46L430

Hi,

if I am using FP calculations in FIQ and IRQ and the FIQ interrupts the IRQ, I get wrong comparisons.
Say I have the following instructions in IRQ:

VCMPE.F32	s16,s17
VMRS		APSR_nzcv,FPSCR

Do I only have to save the registers s0-s31 or do I have to save the register FPSCR, too?

I would use this code to get around my problem:

;start of FIQ routine
VPUSH	{s0-s15}
VPUSH	{s16-s31}

;FP calculations in FIQ

VPOP	{s16-s31}
VPOP	{s0-s15}
;end of FIQ routine

Will this work?

Thanks!

  • 0
    TI__Guru**** 186286 points

    Hi Falk,

    If you don't save those registers, you may get error after back to main code from the IRQ/FIQ ISR if the previous value in those registers are needed. I don't understand why you get error (FPSCR flag) in the ISR?

  • 0 in reply to QJ Wang
    Intellectual 335 points

    Hi!

    I managed to fix the problem with this asm file:

    	AREA SVC_Area, CODE, READONLY
    PRESERVE8

	export	_saveVfp
	export	_loadVfp
		
_saveVfp

	VMRS    r0, FPSCR
	PUSH	{r0}
	VPUSH	{s0-s15}
	VPUSH	{s16-s31}
	
	bx    	lr
	
_loadVfp

	VPOP	{s16-s31}
	VPOP	{s0-s15}
	POP		{r0}
	VMSR    FPSCR, r0
	
	bx    lr
	
	END

    Inside an interrupt, which uses float, I call "_saveVfp()" at the beginning and "_loadVfp()" at the end.
    It works, but is there a more simple solution?

    Regards,

    Falk

  • 0 in reply to Falk Sobe
    TI__Guru**** 186286 points
    Hello Falk,

    Thanks for posting your solution. They look good.

    IF the VFP is enabled and floating point is used, the calling function should save S0~S15 of the VFP registers when it calls the ISR and restore them when the program exits the ISR. The other registers (S16~S31, and FPSCR ) are not preserved before the ISR call, the calling function must save them if their values need to be preserved.

    I just did a test with RTI compare 0 ISR (IRQ) which contains floating operation. I didn't see the ISR preserve the VFP registers.